Wed February 02, 2011 By: Dipti Priya

Mam please explain me the working of transistor as a switch and amplifier.I am not able to understand it from book.Plz help me

Expert Reply
Wed February 02, 2011
Some transistor circuits are CURRENT amplifiers, with a small load resistance; other circuits are designed for VOLTAGE amplification and have a high load resistance; others amplify POWER. The larger collector current IC is proportional to the base current IB according to the relationship IC =βIB , or more precisely it is proportional to the base-emitter voltage VBE . The smaller base current controls the larger collector current, achieving current amplification.The analogy to a valve is sometimes helpful. The smaller current in the base acts as a "valve", controlling the larger current from collector to emitter. A "signal" in the form of a variation in the base current is reproduced as a larger variation in the collector-to-emitter current, achieving an amplification of that signal.

When the switch is closed, the transistor goes into saturation, due to which the voltage at the output is zero.

When the switch is open, the transistor goes into Cut-off State, as there is no base current. Hence the collector current drops to zero. Now, as there is no current flowing through resistor, the collector voltage rises to the supply voltage (5V) and voltage at the output is 5V. 

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